1. Field of the Invention
The invention relates generally to a control apparatus and method for a motor drive system. More specifically, the invention relates to a control apparatus and method for a motor drive system that converts a direct-current voltage to an alternating-current voltage using an inverter to drive and control an alternating-current motor.
2. Description of the Related Art
A motor drive system that converts a direct-current voltage to an alternating-current voltage using an inverter to drive and control a three-phase alternating-current motor is commonly used. Various control modes for appropriately controlling the torque output from the alternating-current motor to a target torque (a torque command value) in such a motor drive system have been proposed.
For example, Japanese Patent Application Publication JP-A-2005-124359 describes a method in which a current signal estimated by an observer having a correction term, which is used to correct an error between the actual speed and the estimated speed, is used in current feedback in the current control for a synchronous motor. In the current control, a torque voltage command is prepared based on a torque current command value (a command value for a d-axis current Id) and a current feedback signal. According to Japanese Patent Application Publication JP-A-2005-124359, a voltage applied to each phase of a synchronous motor (an alternating-current motor) is generated, according to the above-mentioned current feedback, by a switching operation performed in an inverter according to the pulse-width modulation (PWM) control. In order to efficiently drive the motor, PWM control is usually performed by performing vector control.
However, only limited line voltage of the fundamental-wave is obtained by the PWM control. Japanese Patent Application Publication JP-A-11-299297, therefore, describes switching the motor control mode, depending on the situation, between the normal PWM control mode and the control mode in which a rectangular-wave voltage with the PWM duty ratio fixed to the maximum value is applied to a motor and the torque output from the motor is controlled by controlling the phase of the rectangular-wave voltage (the rectangular-wave voltage control mode).
According to the motor control mode described in Japanese Patent Application Publication JP-A-2005-124359, the control can be performed more efficiently, because an electric current is estimated accurately and an induced voltage is compensated for. However, the motor control mode is fixed to the PWM control mode. Accordingly, it is difficult to always obtain a high output regardless of the operating states of the motor.
In order to address such inconvenience, Japanese Patent Application Publication JP-A-11-299297 describes switching the motor control mode between the PWM control mode and the rectangular-wave voltage control mode. With such configuration, some measures need to be taken so that fluctuations do not occur in the torque output from the motor when the control mode is switched. However, there is no description concerning such measures in Japanese Patent Application Publication JP-A-11-299297.